1. Field of the Invention
This invention relates to novel plasticized polymer compositions. More particularly, this invention relates to plastisol compositions comprising 1) a dispersion grade of a homopolymer or copolymer of vinyl chloride and 2) as the plasticizer, a mixture comprising a) the diester derived from a diol containing from 2 to 12 carbon atoms and an aromatic monocarboxylic acid, b) the diester derived from said diol and an aliphatic monocarboxylic acid and c) a mixed ester of said diol and said aromatic and aliphatic monocarboxylic acids. The aromatic monocarboxylic acid is benzoic acid or a substituted benzoic acid. Small concentrations of the monoesters of either or both of the monocarboxylic acids can also be present.
This invention also relates to plasticized polymer compositions containing the plasticizers of this invention in combination with organic polymers other than polyvinyl chloride.
Preferred plasticizers of this invention are low viscosity liquids at 25° C. and are effective primary plasticizers for plastisols containing vinyl chloride polymers.
2. The Prior Art
Ester compositions prepared by reacting a diol with a mixture of benzoic acid and an alkanoic acid and the use of these ester compositions as plasticizers for vinyl chloride polymers that are fabricated as molten materials is taught in a number of patents, including U.S. Pat. Nos. 2,585,448, 2,637,714 and 2,700,656 to Emerson and Longley.
Homopolymers and copolymers of vinyl chloride are available in two forms that are defined by both average size of the polymer particles and behavior of the polymer in the presence of a plasticizer. One form, which is described in the aforementioned prior art, is referred to as a “general purpose” or “suspension grade” polymer. When this type of polymer is combined with a liquid plasticizer, the plasticizer is absorbed into the interior portion of the polymer particles and the product is a dry powder at 25° C. The polymer can prepared by suspension polymerization, and typically exhibits an average particle size of 150 microns. The plasticized polymer is typically fabricated by heating the particulate form of the polymer to form a molten material using techniques including, but not limited to, hot roll milling, extrusion, injection molding and calendaring. All of the vinyl chloride polymers described in the aforementioned patents to Emerson et al. are suspension grade polymers.
A second type of vinyl chloride polymer is used in the preparation of plastisols. This type of polymer exhibits an average particle size of less than 10 microns. Emulsion polymerization is one technique for preparing this type of polymer. When this form of polymer, often referred to as a dispersion grade polymer, is treated with a liquid plasticizer, the plasticizer remains exterior to the polymer particles and the resultant liquid/solid suspensions are referred to as plastisols. The flow characteristics exhibited by these plastisols range from those of liquids to non-drip paints.
End use applications for plastisols include but are not limited to coatings and films. Following application of the plastisol to a substrate, the plastisol is heated to melt the suspended polymer particles and fuse them together to form a unitary article.
Particle size can be measured using known techniques including, but not limited to, microscopic examination of the particles and the use of a series of sieves of gradually decreasing mesh size.
In summary, suspension and dispersion grades of vinyl chloride polymers are characterized and distinguished from one another both by their average particle size and their behavior in the presence of a liquid plasticizer.
U.S. Pat. No. 3,370,032 to Potter discloses a plasticizer for vinyl chloride plastisols. The plasticizer is obtained by reacting 1) a dihydroxyl-substituted ester derived from 2-hydroxymethyl-2-methylpropanoic acid and 2-hydroxymethyl-2-methylpropanol with 2) a mixture of aliphatic and aromatic carboxylic acids. The aromatic acid constitutes from 30 to 70 weight percent of the acid mixture used to prepare the plasticizer.
In accordance with the teaching of this patent, the advantage of using the diester of the mixed aromatic/aliphatic acids in place of the corresponding dialiphatic acid ester is the higher solvating ability of the mixed ester for the vinyl chloride polymer, resulting in a lower “flux temperature” exhibited plastisols containing the mixed benzoate/alkanoate ester as a plasticizer relative to the flux temperature of plastisols containing the dibenzoate. The patent defines “flux temperature” as the minimum temperature to which the mixture of polymer and plasticizer must be heated to allow the particles of plasticizer suspended in the plastisol to fuse to form a unitary solid material such as a film.
The present invention resides in the discovery that some of the mixed ester plasticizers disclosed in the aforementioned patents to Emerson and Longly, which are taught as being useful only for suspension grade vinyl chloride polymers, can be substituted for the structurally unrelated ester compositions of the Potter patent in plastisols. The resultant plastisols exhibit unexpected advantages that include but are not limited to relatively low freezing point of the plasticizer and low viscosity of the plastisol in the absence of excessive softness in shaped articles such as films and coatings prepared using the plastisol. These advantages are unexpected based on the lack of specific or implied teachings concerning these advantages in any of the aforementioned Potter or Emerson et al. patents.
When used at levels that achieve the desired reduction in plastisol viscosity, many prior art plasticizers excessively soften shaped articles prepared from the plasticizer. A commonly used method for avoiding this excessive softening is to reduce the concentration of plasticizer and include a volatile organic solvent that, like the plasticizer, reduces the viscosity of the plastisol, thereby facilitating processing of the plastisol, but is volatilized or otherwise removed during fusing of the polymer particles in the plastisol to form a shaped article. The recovered diluent must be recovered for reuse or disposed of in an environmentally acceptable manner.